The job has proved somewhat bothersome, due mainly to a leaky ammonia line which delayed removal of the failed unit last Saturday. At the second attempt on Wednesday, Wheelock managed to disconnect the line from the module using brute force.

With the old pump now removed, Caldwell Dyson and Wheelock can bolt in the new kit, which is currently sitting on a stowage platform outside the ISS's Quest airlock.

They're due to start Monday's EVA just before 11:00 GMT. Wheelock will be making his sixth spacewalk, while Caldwell Dyson will be clocking up her third. ®